1. Field of the Invention
This invention relates to fuel filtering and more particularly relates to filtering fine particles in a fuel filtering system.
2. Description of the Related Art
Meeting government mandated emissions standards for modern engines necessitates the use of sophisticated fuel injection systems. For example, aftertreatment systems may require advanced fuel delivery capabilities such as post-injection of fuel, and combustion recipes may require multiple injection events and/or shaped fuel injection events. Fuel system components, including fuel injectors and fuel injector ports, may exhibit poor durability and performance over time when the fuel supply contains small abrasive particulates. Previous engine fuel systems have operated sufficiently with particulates in the fuel less than about 10 microns in size. Modern high pressure fuel systems have closer tolerances and are less tolerant to particles below about 5 microns, often requiring particulate filtration down to 3 microns or lower. While fuel filters have been shown to achieve the screening of particulates down below two microns in size under laboratory conditions, fuel filters often show lower performance as installed in an application. Fuel filters also show a significant increase in particulate count through the filter after moderate degradation and aging of the filter.
There are several considerations to account for when selecting the mounting location for a particulate fuel filter. Manufacturers of engines, including diesel engines, often sell engines to an original equipment manufacturer (OEM) who then installs the engines into vehicle bodies and prepares those vehicles for delivery to a vehicle dealer. To ensure the broadest and simplest application of a given engine installation, manufacturers of engines couple vital equipment, like fuel filtration equipment, to the engine. However, fuel filters mounted on a vehicle, and especially directly on an engine, have exhibited significantly lower filtering performance than identical filters in a laboratory test condition. Nevertheless, mounting the fuel filters on the engine directly is desirable to provide a known and testable environment for the placement of engine components, as the vehicle configurations for a particular engine model are likely to vary widely. Further, OEMs prefer that engine systems require as little interaction with the vehicle as possible, and determining filter mounting locations for each vehicle adds to the engine integration burden.
Engines used in non-vehicle applications also install fuel filters in vibrational contact with the engine. For example, a pre-filter on an industrial application may be installed on a skid frame that is vibrationally in direct contact with an engine, and a final fuel filter that is mounted on the side of the engine. The pre-filter may be designed to filter small particles—for example particles larger than about 7 microns, while the final fuel filter may be designed to filter particles larger than 3-4 microns. Both of these filters may suffer from reduced filtration efficiency (i.e. increased inefficiency) relative to a test performance and/or a new filter performance, resulting in greater wear and earlier failure of fuel system components than initially estimated.
High performance fuel filters present other engine design challenges as most fuel filters continue to be rated according to tests developed for earlier, less sensitive filters. The in-use (in the field under normal operating conditions) filtering efficiencies observed for fine particles often do not match the testing efficiencies, causing injector failures and other problems much sooner than should be expected. Because modern filters of fine particulates operate at very high efficiencies, a modest degradation can dramatically increase particle counts passing through the fuel filter. For example, if a filter operates at 99% efficiency, but degrades to 97% efficiency after moderate use, the particle count through that filter will triple. The excess particulates in the fuel supply may cause injector degradation and fuel quality fluctuations. The lower filtering efficiency, in-use and after moderate degradation or aging, observed with fine particles may be such that a filter passes testing, and yet regularly fails in-use. Enhancing the efficiency of fine particle filtering, for example in fuel filters below about 10 micron filtering, will enhance the matching of laboratory tested filter results to in-use filter results, make fuel filters more robust to degradation through use and aging, and generally increase the capability of fuel filters to filter particles in the low micron particle size range.